The enzymes involved in iodothyronine metabolism in liver, brain, and pituitary will be purified and their actions analyzed. Regulation of iodothyronine deiodinase activity in liver, brain, adenohypophysis, and pineal by hormones and various neural influences will be investigated. We will investigate control mechanisms regulating the nyctohemeral rhythm of Type-II 5'-deiodinase which we have discovered in pineal and adenohypophysis and evaluate its possible physiologic role.